A technique for executing vector control on an electric motor by using an inverter has been widely used (see, for example, Patent Literature 1 mentioned below). The vector control for the electric motor is a technique for managing and controlling a magnetic flux component and a torque component separately in a rotary coordinate system, and has been used also in control of an electric vehicle in recent years.
In an inverter for driving an electric vehicle, an asynchronous PWM mode, in which the carrier frequency does not depend on the frequency of an AC output voltage command, is used in a low-speed region. Thereafter, when an upper limit of a modulation factor obtained by asynchronous PWM control is exceeded, a synchronous PWM mode, in which the carrier frequency is an integral multiple of the frequency of an AC voltage command (for example, a synchronous three-pulse mode), is used, and a one-pulse mode is used in a high-speed region in which an output voltage is saturated and fixed to a maximum value.